The present invention relates to a funnel for use in color cathode ray tubes.
Referring now to FIG. 3, the glass components of a color cathode ray tube includes a panel 1 on which a picture is projected and a funnel 3 that has a neck portion 2 where an electron gun is inserted.
The panel 1 and funnel 3 are sealed together through a frit glass 5 between the confronting seal end surfaces 4 thereof. For this sealing, the funnel 3 is held with the seal end surface 4 thereof facing up, and a slurry of mixture of an organic binder (so called “vehicle”) and a crystalline frit glass is applied on the seal end surface 4. After the slurry is dried, the panel 1 is mounted, with the seal end surface 4 thereof facing down, onto the funnel 3 and then heated for sintering.
During this sealing process using a frit glass, a low-viscosity slurry of frit glass 5 should be uniformly applied on the seal end surface 4 of the funnel 3 as much as possible in view of coating performance and high sealing strength.
However, since the seal end surface 4 of the conventional funnel 3 is in the state of mirror-finished, when the slurry of frit glass 5 is applied much thereon, the frit glass 5 is not held on the seal end surface 4 but spills out from the seal end surface 4. As a result, the sealing portion has a so-called re-entrant shape where the frit glass 5 makes a small angle a, with the outer surface of the funnel 3 in the vicinity of the seal end surface 4. For this reason, the sealing strength between the panel 1 and the funnel 3 may be degraded, as a result, a fracture of the glass bulb may occur in the subsequent evacuation process during cathode ray tube manufacturing due to the degradation of the sealing strength. Thus, the applicant of the present invention proposed in Japanese Patent Publication No. Hei. 7-95431 a funnel for a cathode ray tube in which a seal end surface thereof was a rough surface having a plurality of micro dimple-like portions, so as to prevent a frit glass from spilling and to secure the frit glass on the seal end surface. As a result, it was possible to obtain a preferable sealing shape.
Meanwhile, further improvements are wished with regard to remains of micro foreign materials on the seal end surface and micro air bubbles in the sealing portion. Further, a funnel for a cathode ray tube having more excellent higher sealing strength is wished.
Specifically, in the funnel having the seal end surface of which is the rough surface, micro foreign materials such as organic materials, carbon or micro metallic powder adhere to the seal end surface during manufacturing or packing processes, and may remain in the dimple-like portions on the seal end surface even after the funnel has undergone the cleaning process. In other cases, since the frit glass applying on the seal end surface does not completely fill in the dimple-like portions during the sealing process, air remains in the dimple-like portions. When a high voltage is applied to a cathode ray tube using a glass bulb in which a funnel and a panel is sealed together in the state of micro foreign materials, dirt or the like remaining on the seal end surface, electric charge may concentrate on the micro foreign materials or dirt are adhered. In the case of air remaining in the dimple-like portions, micro bubbles may be generated in the sealing portion. As a result, it is not able to obtain a cathode ray tube having a predetermined sealing strength and performance. For sealing the panel and the funnel made of glass with each other, these glass components are heated and sintered usually at temperatures of 450° C. or lower so as to minimize deformations thereof. In this process, ZnO—PbO—B2O3 crystalline frit glass, for example, is employed to prevent softening of the sealing portion including the frit glass in the subsequent evacuation process (maximum temperature is about 350° C.) during a cathode ray tube manufacturing.
This crystalline grit glass begins softening and flowing at about 400° C. by heating, and crystals of 4PbO·B2O3 and 2ZnO·PbO·B2O3 are precipitated in the frit glass, and then the frit glass changes into a crystalline glass superior in heat-resistance thermal expansion coefficient which is compliant with that of the funnel glass (approximately 100×10−7/°C.) via a 30-60 min sintering at about 440° C. In the boundary between the frit glass and funnel glass, the frit glass reacts with the glass of the seal end surface of the funnel while crystallizing and erodes the glass by the wedging effect to form a chemical bonding for sealing. The adhesive strength of frit glass is determined by the degree of reaction and erosion between the frit glass and the funnel glass. For examining the degree of adhesion between the funnel glass and the frit glass, we may observe the state of formation of grains like rice-grains (hereafter, called “R grains”) formed on the surface of the seal end surface, that is the boundary between the funnel glass and the frit glass, due to the reaction of the crystals of frit glass, after solving away the ingredients of the frit glass with strong acids like nitric acid. For providing a high sealing strength, the R grains must be formed finely with chaining structure on the surface of the funnel glass. The R grains present in almost needle shapes, approximately 5-30 μm in major dimension and approximately 1-10 μm in minor dimension. The fret glass after sintering presents the state of crystallized glass containing crystalline and non-crystalline phases. Since the ratio of the crystalline volume dependent on the state of formation of the R grains is a primary factor that determines the thermal expansion coefficient of the frit glass, it is important that the R grains are formed finely with chaining structure as much as possible, in order that the frit glass has an appropriate thermal expansion coefficient corresponding the funnel glass. Generation of the micro bubbles in the sealing portion becomes to be a great impediment for the formation of the R grains during the sealing process, so that there arises such problem that an appropriate expansion coefficient of the frit glass as well as high sealing strength and good sealing shape can not be obtained.